Firefox: If you liked the look of Zen Habits' Minimalist Gmail adaptation, but don't usually roll with Greasemonkey scripts, a coder has put together a single Firefox extension that can hide header, footer, and sidebar elements for an elegant look.
After installing the extension, you'll get a "Minimalist Gmail" menu in the upper-right corner of Gmail. Click it, and you'll have the option to hide whatever pieces of the Gmail interface you want. They'll still be accessible by keyboard shortcuts, of course, and the header itself is accessible through a tiny arrow placed in the upper-left corner. Here are your options with the Minimalist Gmail menu:
Gina's own Better Gmail 2 extension can hide some of the same elements as Minimalist Gmail, along with adding other features that might work nicely in conjunction with the skinning extension. Minimalist Gmail is a free download, works wherever Firefox does.